The use of bubbling fluidized bed and circulating fluidized bed systems in the burning of carbonaceous materials to generate steam from heat exchangers disposed within fluidizing reactors is well documented throughout the literature. The steam is used for electric power generation, process heat, space heating, or other purposes.
A typical bubbling fluidized bed system is described in U.S. Pat. No. 4,301,771 (Jukkola et al.). Such systems generally include an air distribution chamber (usually called a windbox), a bubbling bed furnace, and a convection bank. The windbox receives air for fluidization of the feed material and distributes it uniformly throughout the bottom of the reactor chamber. The reactor chamber consists of a bubbling bed in the lower section and a freeboard in the upper section, all encased in a water-cooled membrane wall. The membrane wall may provide a part or all of the required heat transfer surface area for heat recovery. Additional heat transfer surface area, if necessary, can be provided by in-bed tubes. The gases exhausted from the reactor chamber enter a convection bank for further recovery of sensible heat contained in the gas and the entrained solids. Some of the entrained solids ma be captured in the convection bank and returned to the reactor chamber primarily for enhanced sorbent utilization and bed particle size control.
The bubbling bed process has some similarities to the circulating fluidized bed process, such as the use of inert bed material and the fluidization of the bed with air. That is, fluidizing air is introduced to the bottom of the bubbling bed and agitates the inert solids to create turbulent motion of the bed material. Air, upon being introduced through small orifice holes, creates small bubbles. The bubbles coalesce to bigger bubbles and rise through the inert bed due to buoyancy forces. The bubbles explode at the surface of the bed and splash the bed particles. Some of the splashed particles are elutriated and entrained in an upward flow of the moving gas stream. Relatively low gas velocity during the operation limits the amount of entrained solids. Because of the limited quantity of entrained solids in the freeboard there is a sudden change in solid concentrations across the surface of the bed. As a result, the bubbling or dense bed can be clearly distinguished from the freeboard due to the discontinuity in solid density gradients.
Fuel is introduced to the bubbling bed where it is combusted with sufficient amount of air introduced at the bottom of the bed. Most of the burning takes place in the bed or its immediate vicinity. Upon being entrained in the up flowing gas stream, however, unburned combustibles tend to escape the system without further burning. Heat transfer takes place in both the bubbling bed and in the freeboard area during combustion. A higher heat transfer rate is experienced in the bubbling bed because of extensive contact between solids and heat transfer surfaces, caused by the turbulent motion of the bed. The bed is maintained at a constant temperature during the operation owing to an extremely high heat reservoir of the bed. However, in the freeboard gas temperatures decrease along the height of the freeboard. In any cross-section of the freeboard the rate of heat transfer is higher than the rate of heat supplied or generated in the section. Therefore, the gas is cooled as it moves upward. The gas temperature at the outlet of the freeboard can be 300.degree. to 400.degree. F. lower than the bed temperature.
Some of the disadvantages associated with the bubbling bed process are: relatively small amount of heat transfer occurs in the freeboard region, the flue gas cools down as it traverses the freeboard region resulting in higher carbon monoxide emission, all of the combustion air is introduced at the bottom of the bed, and reduced combustion efficiency due to high carbon monoxide emission.
In order to overcome the disadvantages and inefficiencies of the bubbling fluidized bed process, the circulating fluidized bed process was developed. Circulating fluidized bed systems involve a two phase gas-solids process which promotes solids entrainment within the up flowing gas stream in the reactor chamber and then recycles the solids back into the reactor chamber with a high rate of solids circulation. The rate of solids circulation in the circulating fluidized bed process is about 50 times that of a bubbling bed process. Moreover, circulating fluidized bed systems typically use elongated reaction chambers which increase solids residence time, thus increasing carbon combustion efficiency, increasing heat transfer and decreasing carbon monoxide emission levels.
Various examples of known circulating fluid bed systems are described in U.S. Pat. No. 4,165,717 (Reh et al.) and U.S. Pat. No. 3,625,164 (Spector), and an article by A. M. Leon and D. E. McCoy, presented at the First International Conference on Circulating Fluidized Beds, Halifax, Nova Scotia, Canada (Nov. 18-20, 1985), entitled "Archer Daniels Midland Conversion to Coal".
Of particular interest is the Leon et al. article which involves the use of circulating fluid bed technology to generate steam from the burning of carbonaceous material. It discloses a circulating fluidized bed boiler which utilizes both a dense or "bubbling bed" and a dilute "fast" bed. The bubbling bed is at the bottom of the combustor with the dilute phase above. The operation with both a dense and dilute phase is achieved by permitting some of the combustion air to bypass the dense bed and enter at the bottom of the dilute phase. The dense bed and the dilute phase are accomplished by passing some of the combustion air around the dense bed. The bypassed or secondary air enters above the dense bed at various levels.
The present inventor has discovered that the circulating fluidized bed boiler has a number of disadvantages which can be classified into the following categories, i.e., control and erosion.
The problem of control arises when the circulating fluidized bed boiler is used to burn coal or coal wastes. During the burning of coal or coal wastes the temperature and excess air in the combustor must be maintained at specific values in order for the SO.sub.x, NO.sub.x and CO emissions to remain satisfactory during low loads. That is, it is not acceptable when utilizing the conventional circulating fluidized bed boilers to deviate from predetermined values of temperature and excess air once the load factor drops to below 70%.
The second disadvantage which arises during commercial operation of conventional boilers is severe erosion of the boiler's heat exchange tubes, especially those tubes which line the sidewalls and roof of the combustor. It is believed that the erosion is caused by the high velocities necessary to achieve satisfactory heat transfer. It has been observed that some tubes can wear away and fail after only 1,000 hours of operation, particularly those tubes located in the roof and corners of the combustor. Various palative methods have been proposed to combat erosion, such as, fins, metal spray, studs and covering with refractory (see U.S. Pat. No. 4,714,049).
The present inventor has developed a unique bubbling fluid bed boiler with recycle which incorporates the advantages of both the circulating fluid bed and bubbling fluid bed systems, while overcoming the operational control and heat exchange tube erosion problems associated with those conventional systems. The present invention overcomes the aforementioned disadvantages by designing a circulating fluid bed boiler which includes a reactor chamber with a lower combustion region comprising a plurality of fluid bed zones having internal heat exchange means disposed within at least some of the zones. The multiple fluid bed zones are disposed in the dense or bubbling bed of the reactor chamber and are capable of controlling emission and reducing tube erosion, while maintaining satisfactory heat transfer levels.
Additional advantages of the present invention shall become apparent as described below.